Aromatic Substitution. XIII.la Comparison of Nitric Acid and Mixed ...

3688 G. A. OLAH. S. J. KUHX, S. H. FLOOD AND J C ETANS Irol. s4
ene sulfone) and with mixed acid nitrations in the
same solvent systems.
Nitration with Nitric Acid in Organic Solvents.---
IYith nitric acid (used both as solvent and as
nitrating agent) Ingold has found that nitrations
are first order with respect to aromatic compounds.
Allthough, according to Kaman spectroscopic ob-
servations, :: anhydrous nitric acid contains ap-
proximately lY0 of nitronium ion, solutions of
nitric acid in organic solvents such as nitromethane
contain no detectable amount of nitronium ion.
Ingold, Hughes and co-workers have found that
sufficiently reactive aromatic compounds (benzene,
toluene, ethylbenzene, etc.) exhibit, in nitrations in
nitromethane solutions, zeroth order kinetics, ;.e.,
the reaction rates are independent of the nature or
concentration of the substrate indicating that the
aromatic compound takes no part in the rate-de-
termining step. This step must therefore be limited
to the nitric acid (with the possible assistance of the
solvent) and was found to be the formation of X02+.
The necessary- condition in the formation step of
the nitronium ion is that the rate of the recombi-
nation of nitronium ion with water should be much
slower than the rate of reaction of the nitronium
ion with the aromatic compound. In nitro-
methane solution there will be at the start of the
reaction a minute concentration of water arising
from the ionic self-dehydration of the nitric acid.
During the reaction this will be increased, but the
water concentration will always be much less than
that in the partly aqueous solution. In nitro-
methane solution the zeroth order rate was ob-
served by Ingold and Hughes in almost all cases
involving compounds with reactivities equal to or
greater than that of benzene. The addition of
small quantities of water to nitromethane has little
effect on the zeroth order rate. This is expected,
since water does not enter into the pre-equilibrium
step and therefore does not reduce the concentra-
tion of the nitracidium ion. IThen more water is
added, a point is reached where the water competes
with the aromatic compound for the limited supply
of the nitronium ion. The kinetics then change to
a first-order form. 16
The relative reactivity of toluene and benzene in
competitive nitration with nitric acid in nitro-
methane was found by Ingold" to be 21, using
a dilatometric technique. Employing gas-
liquid chromatography as an analytical method,
the nitric acid nitration of benzene. toluene, ethyl-
benzene, $-xylene and mesitylene were cornpared
in competitive experiments (Table I). The results
are in excellent agreement with Ingold's data for
the toluene : benzene reactivity ratio and also
establish high substrate selectivity and the usual
isomer distributions in the nitration of the other
alkylbenzenes.
Nitric acid nitration of toluene and benzene in
acetic anhydride and acetic acid solutions was in-
vestigated by Ingold and Hughes during their
sTTstematic study of nitrations." Knowles.
Korman and RaddaI6 redetermined relative rates
(I.?) E. 13. Hughes, C K. Ingold and R R. Pearsun, .I CIi~?ii. .So(.,
4957 (1958).
(It!) J. R Knowlep. R 0 C. Sorman and C,. K. RadBa, J. Chriii
Sac., 48%